Squashed 'plugin/floo/common/' changes from 39819ce..808933b

808933b Don't send None.
f88dfa0 Add req_id/res_id support.
170da1d Don't md5sum twice. Mention if there was an error calculating checksums for a buffer. Add more logging to on_patch.

git-subtree-dir: plugin/floo/common
git-subtree-split: 808933b759228cbc74f4ded2b695d5a3ba925124
Merge commit '5c2215efcdde53bd0c638a020df524b2e0abd1c2' into floorc_json